Flame responsive override control system including electric servomotors



Feb. 24, 1959 L. M. oBERLlN FLAME RESPON 2,874,541 YSTEM SIVE OVERRIDE CONTROL INCLUDING ELECTRIC SERVOMOTOR 2 Sheets-Sheet l Filed Dec. l5, 1952 AMPLIFIER T I I SERVO RECEIVER faz THROTTLE I I I l L faz THROTTLE AMPLIFIER RECEIVER INVENTOR. L.M.OBERLIN ATTORNEYS Feb. 24, 1959 L. M. OBERLIN 2,874,541

FLAME RESPONSIVE OVERRIDE CONTROL SYSTEM INCLUDING ELECTRIC SERVOMOTORS Filed DSC. l5, 1952 2 SheebS-Shee. 2

THROTTLE sauf INVENTOR. 3X I .MOBERLIN ATTORNEYS United States Patent O FLAME RESPONSIVE OVERRIDE CONTROL SYS- TEM INCLUDING ELECTRIC SERVOMOTORS Lyman Malcolm Oberlin, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application December 15, 1952, Serial No. 325,955

' 12 Claims. (Cl. 60--39.28)

This invention relates to control mechanism. In one specific aspect it relates to control mechanism which is actuated only during such times as the variable being regulated exceeds predetermined limits.

In various types of burners it is desirable to control the rate of combustion. This is especially true in jet-type engines wherein it is important to prevent flame from extending beyond the normal combustion chamber because flames contain reactive gases which can erode Very rapidly some metallic and even some non-metallic parts of the engine, particularly when these parts receive an intermittent bath of flame. It has been found that if the flame front extends beyond the combustion chamber considerable damage may result from flame vimpinging upon elements not capable of withstanding high temperatures and/or chemical reactions of these reactive gases present and created by the flame. A second danger is the possibility of an explosion taking place either within or outside the combustion chamber.

One particular example wherein such excessive flame fronts must be prevented is found in turbojet aircraft engines. in such engines air is compressed and passed to a combustion chamber where it is mixed with fuel to support combustion. The resulting products of combustion then pass through a turbine which has a driving connection with the compressor. After passing through the turbine the combustion products are ejected through an outlet nozzle, thereby producing a reaction which provides the forward propulsive thrust. It is important to prevent excessive flame fronts from impinging upon the turbine blades because these blades normally are constructed of materials which cannot withstand prolonged contact with flame. In many of these engines additional fuel is mixed with the gases from the combustion chamber after they leave the turbine, and the second mixture then passes through a grid-like structure to a second combustion chamber where further burning takes place. If the supply of fuel to the secondary mixing zone is excessive flashback may occur. That is, flame may be present in the secondary mixing chamber upstream of the flame holder. it then becomes important to be able to detect the presence of flame in this chamber so that the condition may be remedied as by regulating the fuel supplied to the engine.

In accordance with this invention a control system is provided which comprises a pair of servo transmitters together with a single servo receiver. One of the servo transmitters is energized by the normal control of the variable under consideration, which in an aircraft engine can be the manual fuel throttle. The second servo trans mitter is actuated by a detecting element which senses any abnormal condition which must be remedied. Thus, the second transmitter is in effect an override control. Switching mechanism is provided which is energized by the detecting element to disconnect the first servo transmitter from the receiver whenever the override control is necessary and at the same time togconnect the second transmitter to the receiver. Whenever conditions are such that the override control is no longer required, the switching mechanism returns the first transmitter to the receiver which once again allows normal control.

Accordingly, it is an object of this invention to provide control mechanism including switching means whereby a plurality of servo transmitters are connected selectively to a single servo receiver in response to the magnitude of the variable being controlled.

Another object is to provide automatic control mechanism which is actuated only when the variable being controlled exceeds predetermined limits.

Another object is to provide apparatus for controlling flame fronts in aircraft engines.

A further object is to provide control apparatus to prevent ffame from impinging upon the turbine blades in a turbojet aircraft engine.

A further object is to provide control apparatus to prevent flashback in jet engines.

Various other objects, advantages and features of this invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a schematic View, shown partially in section, of flame detection and control mechanism employed in conjunction with a turbojet engine;

Figure 2 is a view, shown partially in section, of ame detection and control mechanism employed in a second manner in conjunction with a turbojet engine;

Figure 3 shows an appropriate flame sensing probe for use as the detecting element in Figures l and 2; and

Figure 4 is a schematic electrical circuit diagram of the control system of this invention.

Referring now to the drawing in detail and to Figure 1 in particular, there is shown a schematic representation of a turbojet aircraft engine which comprises a casing or housing it) of essentially circular cross-section having an air intake 11 and a discharge nozzle 12. Proceeding from the inlet end to the outlet end, casing 10 contains an air compressor 14, a plurality of combustion chambers 15 arranged annularly around the engine at equally spaced intervals, and a gas turbine 17. Air compressed by compressor 14 is utilized to support combustion of a liquid fuel which is injected into combustion chambers 15 through fuel nozzles 16, and the greatly increased volume of resulting heated gases is fed through turbine 17 and thence outwardly through discharge nozzle 12. The purpose of turbine 17 is to drive air compressor 14 by means of a shaft 18 connecting the rotor assembly of turbine 17 with the rotor assembly of air compressor 14. Turbine 17 includes a peripheral set of stationary vanes 20 and a movable set of blades 2i. Located centrally within the tail pipe of the engine is a discharge air regulating plug 23 which is suspended by appropriate structure, not shown.

For satisfactory operation of such a turbojet engine,

the fuel-air ratio in combustion chambers 15 must bel regulated so that the resulting flame is not of sufficient length to impinge upon either vanes 20 or blades 21 of turbine 17 because a flame impinging thereon will reduce the fatigue strength of these metallic vanes and blades. Therefore, means should be provided both to detect the presence of any flame which may impinge upon these vanes or blades and to regulate the fuel-air ratio in the combustion chambers in response thereto to prevent such excessive flame fronts, To provide this control it has been proposed to position a suitable flame responsive element 25 in each of combustion chambers 15 at a posi-4 tion somewhat forward of vanes 20. As is described in greater detail hereinafter, llame detecting elements 25 provide an output electrical signal whenever flame impinges thereon.

The rate of fuel injection into combustion chambers 15 normally is regulated by means of an adjustable valve 28 disposed in a fuel line 29 which communicates with fuel Ynozzles 16. Valve 2S normally is adjusted by the signal from a servo receiver 3@ which in turn is regulated by a servo transmitter 31, the latter being actuated by a hand throttle 32. Movement of hand throttle 32 results in an opening and closing of valve 23 as is desired to increase or decrease the rateof fuel injection into tbe combustion chambers 15. As previously mentioned, it is important that the rate of fuel passed into chambers 15 remain within certain limits such that the flame front is not of sufficient length to impinge upon the elements of turbine 1'?. If for any reason the flame front tends to extend as far as detectors 25, an output electrical signal is provided which isapplied through a capacitor 34 to an amplifier l35, the output signal of which is applied to a second servo transmitter 36. Simultaneously, this output signal from arnpliiier 35 is applied to a switching mechanism 37 kwhich serves both to disconnect transmitter 31 from receiver 3i? and to connect transmitter 36 to receiver 30 in place thereof. Thereafter, the rate of fuel passing through valve 28 is regulated by servo transmitter 36 in accordance with the amplitude of the signal produced by iiarne detectors 25. As soon as the abnormal condition has been remedied and the ilame front is shortened so as not to impinge upon detectors 25, switching mechanism 36 becomes de-energized such that transmitter 36 is disconnected from receiver 30 and transmitter 31 is onceV again connected to receiver 30.

In Figure 2 there is illustrated a turbojet engine of the type wherein an additional quantity of fuel is added to the exhaust gasesV in the region rearward of turbine 17 in order to give added propulsive thrust to the engine. This secondary fuel is supplied to a mixing chamber 4t), through a ring of nozzles 41 connected to a common fuel supply line 42 which has an adjustable valve 43 disposed therein. In this .manner the initial products of combustion are mixed with secondary fuel in chamber 40 and thence pass through a ame holder 44 to a second combustion charnber 45. Flame holder 44 preferably consists of a set of small cones 46 supported in housing 10 by a suitable latice Work. The velocity of the combustible mixture passing through flame holder 44 from mixing chamber 40 is substantially reduced whereby combustion can be maintained. Such combustion is supported Within the cones in secondary combustion chamber 45, it being understood that the velocity of the gases in mixing chamber 40 is too high toV support combustion. From chamber 45 the combustion gases are ejected through nozzle 12 to produce an additional propulsive thrust. in order to detect the condition of flashback in region 4b, a flame detecting element 25a is disposed in the region between fuel nozzles 41 and iiame holder 44. The output signal from flame detector 25a actuates control mechanism identical to that illustrated in Figure 1 to override the manual fuel control whereby the rate of fuel injection into chamber 40 is reduced to eliminate ashback.

In Figure 3 there is illustrated a flame sensing probe which can be employed as elements 25 or 25a to indicate the presence of ame impingng upon the tip thereof. This probe comprises a metallic casing Sil having a metallic center electrode 51 electrically insulated from casing 50 by a plug of ceramic material 52. Electrode S1 is irnbedded in a tip 54 of ceramic material positioned across one end of casing 56 and making electrical Contact therewith. Ceramic tip 54 preferably is constructed of some type of aluminum oxide such as Aiundum cement. This cement is molded in the form of a tip around casing 59 and then sintered.

The operation of the probe as a ame sensing element, which is more fully described in the copending application of D. R. de Boisblanc, Serial No. 220,113, filed April 9, 1951, can be explained in somewhat the following manner: In any given electrical resistance element such as tip 54 having two electrodes 50 and 51 connected thereto, the first of which is grounded, there exists a constant distribution of potential at all points throughout the element. If charged particles such as the ions produced by the reaction of a ame impinge upon the surface of resistance tip 54, the constant potential condition is disrupted, thereby resulting in a small current iiow in the element until all pointsrare once again at the same ground potential. During the time that current is iiowing, the region of resistance element 54 in the neighborhood of electrode 51 exhibits a potential variation with respect to electrode 50 which depends upon the magnitude of the charged particles striking the element and the geometrical relation of the two electrodes. This potential variation, after amplification, can be measured to provide an indication of the condition of flame impinging upon'tip 54 and such phenomenon is utilized in accordance with the present invention to actuate the automatic control mechanism to correct the rate of combustion to reduce the iiame front as required.

The electrical circuitry employed to perform the control functions previously described in conjunction with Figures 1 and 2 is illustrated schematically in Figure 4. In a preferred embodiment of this invention, transmitters 31 and 36 and motor 3ft are represented by like synchros motors. As employed herein. the term synchros refers to a particular type of electrical motor which is well known to those skilled in the art by several trade names including Selsyn, Autosyn, Telegon, and Diehlsen. Mechanically these elements are of typical electrical motor construction and in their simplest form include twopole rotors with slip rings and three phase stato-r windings.

The rotor 6@ of transmitter 31 is mechanically coupled to throttle 32 whereby rotation of throttle 32 provides corresponding rotation of the rotor 6d. The rotor 61 of receiver 30 is mechanically coupled to motor valve 28 whereby rotation of rotor 61 .functions to open or close valve 28. The rotor 62 of transmitter 36 is mechanically coupled to the rotor of a motor 64 which in turn is driven by the output signal from ame detector 25 whereby t-he rotation of motor 64- is proportional to the magnitude of the signal detected by element 25, the latter being proportional to the magnitude of flame impinging thereon. Rotors 60, 61 and 62 each are connected, as shown, directlyV across a two-wire alternating current transmission line 66, 67 which is supplied by a source of alternating current 68. The three stator coils 100, 101, and 102 of receiver 30 are Y-connected and the three output terminals thereof are connected to respective movable blades 70, 71, and 72 of a triple-pole double-throw switch 37. The three stator coils 103, 104, and 1115 of transmitter 31 are likewise Y-connected and the output terminals thereof are connected to respective iirst terminals 70a, 71a, and 72a of switch 37 which is normally biased by a spring 73 in a first position whereby the stator coils of transmitter 31 Vare connected to the stator coils of receiver 3i?, that is, blades 70, 71, and 72 are in engagement with respective terminals 71M, 71a, and 72a. The three stator coils 106, 107, and 108 of transmitter 36 also are Y-connected and the output terminals thereof are connected to respective secondterminals 70b, 71h, and 72b of switch 37 so as to be connected to the stator coils of receiver 30 whenever switch 37 is thrown to its second position. Switch 37 is actuated by solenoid coil Si), the two terminals of which are connected by leads 81 and 32 to the output terminals of amplifier 35 through a diode S4 connected in series therewith. Y

The series connected stator coils '86 and S7 of motor 64 are connected directly across power leads 66 and 67. The series connected rotor coils 89 Yand 90 of motor 64 `are connected across the endY terminals of the secondary windingvof a transformer 91, the end terminals of the primary winding of which are connected across power leads 66 and 67 through a capacitor 93. One output terminal of ampliiier 35 'is connected by a lead 94 to the center tap of the secondary winding of'tr'ansformer 91 and the second output terminal of amplifier 35 is grounded. In the absence of' an output signal from amplifier 35 motor 64 remains at rest because of the balanced voltages in the two sets of coils which are 90 out of phase. Any output signal from amplifier 35, however, disrupts this balance to provide an output rotation of motor 64.

The operation of the control system of this invention should readily become apparent to those skilled in the art. In the absence of an output signal from probe 25, switch 37 remains in its normal first position whereby the stator coils of transmitter 31 are connected to the stator coils of receiver 30. With both rotors 60 and 61 at rest, each stator has induced in it voltages equal and opposite to the voltages of the other such that no current fiows through the stator coils. If rotor 60 is turned by movement of throttle 32 the balance of the induced voltages in the stators is disturbed and a resulting current flows through the stator coils causing rotor 61 to rotate through the same angle as does rotor 60 in order to once again restore a balanced condition. Accordingly, movement of throttle 32 results in an opening and closing of valve 28. However, should flame impinge upon detector 25, an output electrical signal is generated thereby which is amplified and applied to coils 89 and 90 of motor 64 and also to solenoid coil 80 through diode 84. Whenever this output signal is of a predetermined magnitude which represents excessive flame impingement on element 25, diode 34 becomes conductive to allow current to flow through coil 80 to move switch 69 to its second position which disengages transmitter 31 from receiver 30 and which connects transmitter 36 to receiver 30. The rotation of rotor 62 of transmitter 36 is governed by the magnitude of the output signal applied through amplifier 35 to motor 64. In this manner valve 28 is adjusted in accordance with the signal generated by the flame detector to reduce the fuel flow into the engine as may be needed `to reduce the length of flame front. Once this flame front is reduced, the output signal or" detector 25 is reduced sufficiently to de-energize solenoid coil 80 which again returns switch 37 to its first position such that the fuel flow is once again regulated by the manual throttle 32.

The control system of Figure 2 operates in substantially the same manner as that described in conjunction with Figure l. That is, the fuel-air ratio fed to chamber 40 is regulated to prevent flashback.

While this invention has been described in conjunction with the present preferred embodiments thereof, it should be apparent that the invention is not limited thereto. The essence of this invention resides in providing suitable switching means to disengage the normal control transmitter from the receiver whenever the quantity under control exceeds predetermined limits while at the same time substituting automatic control therefor. The exact type of transmitters and receivers employed obviously can vary from those illustrated as can the detecting element. While this control system has been described in conjunction with regulating flame fronts in jet type aircraft engines, it should be apparent that the principle of control embodied by this invention is not necessarily restricted to any particular application, but rather is useful in a Wide variety of control problems.

What is claimed is:

l. lAutomatic override control apparatus comprising in combination, a first servo transmitter, a servo receiver, means to connect said first transmitter to said receiver, means to vary the output of said first transmitter, means to connect said receiver to a variable to be regulated, a detecting element to sense variance of a quantity to be controlled beyond predetermined limits, said quantity being dependent on said variable, a second servo transmitter coupled to said detecting element whereby the outputof said second transmitter represents variance of the quantity being controlled beyond said predetermined limits, switching means to disconnect said first transmitter from said receiver and to connect said second transmitter to said receiver in place thereof when said quantity being controlled exceeds said predetermined limits, said first and second transmitters and said receiver each comprising an induction type motor having a threephase stator and single-phase rotor, the stator of said receiver being selectively connected to the stators of said first and second transmitters, and an alternating current voltage source, the rotors of said receiver and said transmitters being connected across said voltage source so that rotation of the rotor of either of said transmitters results in corresponding rotation of the rotor of said receiver in accordance with the particular transmitter connected to said receiver.

2. Automatic control apparatus comprising in combination, first and second synchros motors serving as servo transmitters, a third synchros motor serving as a servo receiver, means to selectively connect said receiver to one or the other of said first and second transmitters, means connecting said receiver to a variable to be regulated whereby input signals applied to said first and second transmitters results in adjustment of said variable in accordance with the particular transmitter connected to said receiver, the output of said receiver adjusting said variable, and a detecting element to sense variance beyond predetermined limits of a quantity affected by said variable, said first mentioned means being actuated by said detecting element whereby said first transmitter is disengaged from said receiver and said second transmitter is engaged with said receiver when the quantity being regulated exceeds said predetermined limits.

3. Combustion chamber control apparatus comprising in combination, a flame sensing element adapted to be positioned in the region of a combustion chamber to detect the presence of flame, means to vary the rate of combustion in said chamber, a servo receiver coupled to said means to vary thereby to effect adjustment thereof to vary said rate of combustion, a lrst servo transmitter normally coupled to said receiver whereby variance of the signal applied to said first transmitter results in corresponding variance of the output of said receiver and in the rate of combustion taking place within said chamber, means to vary the signal applied to said first transmitter, a second servo transmitter coupled to said flame sensing element whereby the output of said second transmitter is varied in accordance with the output signal of said element, switching means to disengage said first transmitter from said receiver and to engage said Second transmitter with said receiver at such time as the output signal from said element exceeds predetermined limits, said first and second transmitters and said receiver each comprising an induction type motor having a three-phase stator and single-phase rotor, the stator of said receiver being selectively connected to the stators of said first and second transmitters, and an alternating current voltage source, the rotors of said receiver and said transmitters being connected across said voltage source so that rotation of the rotor of either of said transmitters results in corresponding rotation of the rotor of said receiver in accordance with the particular transmitter connected to said receiver.

4. Combustion chamber control apparatus comprising in combination; a flame sensing element adapted to be positioned in the region of a combustion chamber to detect the presence of flame, said flame sensing element comprising a probe having an electrically conductive casing, an electrically conductive electrode positioned within and electrically insulated from said casing, and a tip of ceramic material positioned across one end of said casing and making electrical contact with said casing and with said electrode, said ceramic tip being positioned in the region wherein flame is to be detected; means to vary the rate of fuel supplied to said chamber; a servo receiver coupled to said means to vary thereby to effect adjustment thereof to vary said fuel rate; a first servo transmitter normally coupled to said receiver whereby variance of the signal Y appliedto said first transmitter results in correspondinggarnaal.

variance .of the output of said receiver and in the rate of fuel supplied to said chamber; means to vary the signal applied to said first transmitter; a :second servo transmitter coupled to said flame sensing element whereby the output of said second transmitter is varied in accordance with the output signal of said element; and switching means to disengage said first transmitter from said receiver and to engage said second transmitter with said receiver at such time as the output signal from said element exceeds predetermined limits.

5. in a turbojet aircraft engine including an air compressor, a combustion chamber positioned rearwardly of said compressor and receiving air at one end from said compressor, a gas turbine positioned at the other. end of said combustion chamber and adapted to be driven by heated gas from said combustion chamber, and a drive shaft connecting said compressor to said turbine; flame control mechanism comprising in combination, a llame detecting element positioned in said combustion chamber forward of said turbine to detect the presence of flame impinging thereon, means to vary the rrate of combustion in said chamber, a servo receiver coupled to said means to vary thereby to effect adjustment thereof to vary said rate of combustion, a first servo transmitter normally coupled to said receiver whereby variance of the signal applied to said first transmitter results in corresponding variance of the output of said receiver and in the rate of combustion taking place within said chamber, means to vary the signal applied to said first transmitter, a second servo transmitter coupled to said llame detecting element whereby the output of said second transmitter is varied in accordance with the output signal of said element, switching means to disengage said first transmitter frorn said receiver and to engage said second transmitter with said receiver at such time as the output signal from said element exceeds predetermined limits, said first and second transmitters and said receiver each cornprising an induction type motor having a three-phase stator and single-phase rotor, the stator of said receiver being selectively connected to the stators of said first r and second transmitters, and an alternating current voltage source, the rotors of said receiver and said transmitters being connected across said voltage source so that rotation of the rotor of either of said transmitters results in corresponding rotation of the rotor of said receiver in accordance with the particular transmitter connected to said receiver.

6. In a turbojet aircraft engine including an air compressor, a combustion chamber positioned rearwardly of said compressor and receiving air at one end from said Compressor, a gas turbine positioned at the other end of said combustion chamber and adapted to be driven by heated gas from said combustion chamber, and a drive shaft connecting said compressor to said turbine; liame control mechanism comprising in combination, a liarne t detecting element positioned in said combustion chamber' forward of said turbine to detect the presence of llame impinging thereon, said name detecting element comprising a probe having a metallic casing, a metallic electrode positioned within and electrically insulated from said casing, and a tip of ceramic material positioned across one end of said casing and making electrical contact with said casing and with said electrode, said ceramic tip being positioned in the region wherein llame is to be detected; means to vary the rate of fuel supplied to said chamber; a servo receiver coupled to said means to vary thereby to effect adjustment thereof to vary said fuel rate; a first servo transmitter normally coupled to said receiver whereby variance of the signal applied to said first transmitter results in corresponding variance of the output of said receiver and in the rate of fuel supplied to said chamber; means to vary the signal applied to said first transmitter; a second servo transmitter coupled to said flame detecting element whereby the output of said second transmitter is varied in accordance with the output signal Aof said element; and `switching means to disengage said Vtiret transmitter from said .receiver and kto engage said second transmitter with said receiver at such time as the output signalV from said element exceeds predetermined limits.

7. In a reaction motor of the turbojet type including a turbine, fuel injection means positioned downstream from said turbine, and a flame holder positioned downstream from said fuel injection means; combustion control mechanism comprising in combination, a flame detecting element positioned between said fuel injection means and said llame holder to detect the presence of llame impinging thereon, means to vary the rate of com bustion in said chamber, a servo receiver coupled to said means to vary thereby to effect adjustment thereof vto vary said rate of combustion, a first servo transmitter normally coupled to said receiver whereby variance of the signal applied to said first transmitter results in corresponding variance of the output of said receiver and in the rate of combustion taking place within said chamber, means to vary the signal applied to said first transmitter, a second servo transmitter coupled to said flame detecting element whereby the output of said second transmitter is varied in accordance with the output signal of said element, and switching means to disengage said first transmitter from said receiver and to engage said second transmitter with said receiver at such time as the output signal from said element exceeds predetermined limits.

8. In a reaction motor of the turbojet type including a turbine, fuel injection means positioned downstream from said turbine, and a flame holder positioned downstream from said fuel injection means; combustion control mechanism comprising in combination, a flame detecting element positioned between said fuel injection means and said llame holder to detect the presence vof fiame impinging thereon, means to vary the rate of fuel supplied to said chamber, a servo receiver coupled to said means to vary thereby to effect adjustment thereof to vary said fuel rate, a first servo transmitter normally coupled to said receiver whereby variance of the signal applied to said rst transmitter results in corresponding variance of the output of said receiver and in the rate of fuel supplied to said chamber, means to vary the signal applied to said first transmitter, a second servo transmitter coupled to said flame detecting element whereby the output of said second transmitter is varied in accordance with the output signal of said element, and switching means to disengage said first transmitter from said receiver and to engage said second transmitter with said receiver at such time as the output signal from said element exceeds predetermined limits.

9. r[he combination in accordance with claim 8 wherein said llame detecting element comprises a probe having a metallic casing, a metallic electrode positioned within and electrically insulated from said casing, and a tip of ceramic material positioned across one end of said casing and making electrical contact with said casing and with said electrode, said ceramic tip being positioned in the region wherein flame is to be detected.

l0. Combustion chamber control apparatus comprising a liame sensing element adapted to be positioned` in the region of a combustion chamber to detect the presence of flame, said ame sensing element comprising a probe having a metallic casing, a metallic electrode positioned within and electrically insulated from said casing, a tip of ceramic material positioned across one end of said casing and making electrical contact with said casing and with said electrode, and an amplifier having its input terminals connected to said electrode and said casing, respectively; rst means to vary the fuel supplied to said chamber; a first synchros receiver to actuate said first means; a first synchros transmitter; means to vary the rotation of said first transmitter; a second synchros transmitter; means coupling said second transmitter to said flame sensingtelement so that variance of the output of said amplifier varies the rotation of said second transmitter; and switching means to connect said rst transmitter to said receiver except when the output signal from said amplifier exceeds a predetermined limit, at which time said second transmitter is connected to said receiver, said switching means comprising a solenoid operated switch, means connecting said receiver to each of said transmitters through said switch so that said receiver is connected to said first transmitter when said switch is in a first position and is connected to said second transmitter when said switch is in a second position, a diode, and means connecting the coil of said solenoid operated switch circuit with said diode and the output terminals of said amplier so that said switch is moved to said second position when the output of said amplifier exceeds said predetermined limits.

1 1. Combustion chamber control apparatus comprising in combination, a ame sensing element positioned in the region of a combustion chamber to detect the presence of flame, means to vary the rate of combustion in said chamber, a servo receiver coupled to said means to vary in order to effect adjustment thereof and thereby to vary said rate of combustion, a tirst servo transmitter normally coupled to said receiver whereby variance of the signal applied to said tirst transmitter results in corresponding variance of the output of said receiver and in the rate of combustion taking place within said chamber, means to vary the signal applied to said first transmitter, a second servo transmitter coupled to said ame sensing element whereby the output of said second transmitter is varied in accordance with the output signal of said element, switching means to disengage said first transmitter from said receiver and to engage said second transmitter with said receiver at such time as the output signal from said element exceeds a predetermined limit and to hold said second transmitter in engagement with said receiver until the output signal from said element is within the predetermined limits, said tirst and second transmitters and said receiver each comprising an induction type motor having a three-phase stator and single-phase rotor, the stator of said receiver being selectively connected to the stators of said iirst and second transmitters, and an alternating current voltage source, the rotors of said receiver and said transmitters being connected across said voltage source so that rotation of the rotor of either of said transmitters results in corresponding rotation of the rotor of said receiver in accordance with the particular transmitter connected to said receiver.

12. In a reaction motor of the turbojet type including a turbine, fuel injection means positioned downstream from said turbine, and a ame holder positioned downstream from said fuel injection means; combustion control mechanism comprising in combination, a tlame detecting element positioned between said fuel injection means and said dame holder to detect the presence of ilame impinging thereon, means to vary the rate of fuel supplied to said chamber, a servo receiver coupled to said means to vary thereby to effect adjustment thereof to vary said fuel rate, a rst servo transmitter normally coupled to said receiver whereby variance of the signal applied to said rst transmitter results in corresponding variance of the output of said receiver and in the rate of fuel supplied to said chamber, means to vary the signal applied to said rst transmitter, a second servo transmitter coupled to said iiame detecting element whereby the output of said second transmitter is varied in accordance with the output signal of said element, and switching means to disengage said iirst transmitter from said receiver and to engage said second transmitter with said receiver at such time as the output signal from said receiver exceed predetermined limits, and to hold said receiver in engagement with said second transmitter until the output signal from said element is within the predetermined limit, said rst and second transmitters and said receiver each comprising an induction type motor having a three-phase stator and single-phase rotor, the stator of said receiver being selectively connected to the stators of said first and second transmitters, and an alternating current voltage source, the rotors of said receiver and said transmitters being connected across said voltage source so that rotation of the rotor of either of said transmitters results in corresponding rotation of the rotor of said receiver in accordance with the particular transmitter connected to said receiver.

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